Plant volatiles – An area that keeps on growing

Wednesday, 10 September 2014 at 10:25:AM

From time to time, I scour the scientific literature for interesting examples of where people have used thermal desorption in scientific research – which is great for highlighting some of the latest trends in how people are harnessing the power of modern analytical technology.

As an organic chemist (and former Publishing Editor of the journal Natural Product Reports), plant volatiles particularly interest me, and I’m always keen to see what’s been happening in this small but rapidly growing field. So here’s my choice of the most interesting papers from the last few years where our TD equipment has helped researchers to study plant volatiles.

Using Markes’ UNITY-ULTRA automated thermal desorption system with GC–MS analysis, they found that a set of five aldehydes – hexanal, (E)-hex-2-enal, heptanal, benzaldehyde and octanal – were present in the volatile blends at the stages when the peach and pear trees were most attractive to the moths. The authors suggest that this information should be used when designing chemical attractants for female fruit moths.

This is an example of a ‘tritrophic’ system – that is, one involving three levels of the food chain, and in essence an extension of the plant–herbivore system described above. Such systems are well-studied,* but this one is remarkable because it involves caterpillars being predated by birds, not other insects.

Despite this fundamental difference, the system studied appears to operate in the same way as normal – namely, the apple trees release a distinct set of volatiles when the leaves are attacked by the caterpillars, and these volatiles are used by the birds to locate the trees where a meal can be found.

To help confirm that the birds were using volatile cues, the authors enclosed tree branches with a polymer bag, and flushed the collected vapours onto sorbent tubes, with analysis by Markes’ UNITY thermal desorber and GC–MS. This analysis showed that the infested trees emitted more alpha-farnesene and dodecanal than uninfested trees, but less 1,2,4-trimethylbenzene, oct-1-en-3-ol, methoxy phenyl oxime, non-1-ene and and octan-3-ol.

The authors say that their findings are in line with previous studies on the use of smell by birds to detect prey, and indeed indicate that it may be more important than previously thought. They add that this area of research is exciting because birds eat far more caterpillars than other insects – so the potential benefits for pest control obtainable by breeding plants with enhanced volatile emissions may be correspondingly greater.

In the study, which also involved scientists from Japan and Finland, portable dynamic headspace equipment was used to sample volatiles emitted from branches of 59 sagebrush plants in Sagehen Natural Reserve, California, USA. Volatiles were collected onto sorbent tubes from Markes and then analysed with Markes’ TD-100 automated thermal desorber, in conjunction with GC–MS.

They found that although volatile profiles differed greatly, individual profiles were generally dominated by either alpha-thujone or camphor, and that just two additional compounds (beta-thujone and cis-salvene) were sufficient to classify these two ‘chemotypes’ of sagebrush. Importantly, a separate experiment involving analysis of compounds from seed-raised plants showed that the chemotypes were highly heritable, 80% of ‘children’ having the same chemotype as their ‘mother’.

They then investigated the effectiveness of plant communication by collecting headspace from one plant and incubating it with another plant for 24 hours, before exposing the plant to the open air and so to insect herbivores. They found that the transferred volatiles were more effective at deterring herbivory in the recipient when the plants were the same chemotype. The authors suggest that this knowledge, together with the fact of chemotype heritability, suggests that plant volatiles could help plants to distinguish ‘relatives’ from ‘strangers’, so increasing the value of the information encoded by plant volatiles.

In the work, Brussels sprout plants were reared inside climate-controlled chambers, and headspace volatiles collected passively over 6 hours onto Radiello sorbent samplers. The sorbent cartridges were then desorbed using Markes’ UNITY-ULTRA, with analysis by GC–MS.

They found that 10 weeks’ exposure to doubled CO2 resulted in a number of changes to the volatile profile – namely the appearance of (Z)-hex-3-enyl acetate, dodecane and tridecane, the disappearance of decanal and undecanal, and statistically significant reductions in 10 terpenoids, as well as a 50% fall in dimethyl disulfide.

At the same time, they used wind-tunnel choice experiments to investigate the preferences of winged cabbage aphids for the different plants, and found that the colonisation of plants reared under doubled CO2 was 26% lower than regular plants.

The authors also found that the rate at which carbon dioxide or water entered or left the leaf through the stomata (‘stomatal conductance’) was significantly reduced, and suggest that this, together with possible changes in leaf enzyme activity, could explain the reduction in volatile emissions. They then suggest that this in turn (perhaps in concert with additional cues such as changes in plant size) reduces the attractiveness of the plants to the aphids.

Whatever the underlying mechanism, the authors say that this is one of the very few cases where changes in atmospheric CO2 have been shown to influence herbivory. They also point out that the changes in volatile emissions could have important ecological implications – such as the ability to attract predators of the herbivores (as I’ve described in Luisa Amo’s research above).

David Barden received his Ph.D. in Organic Chemistry from Cambridge University in 2004, and during his time as an editor at the RSC wrote news pieces for Chemistry World on various scientific topics. He is now Technical Copywriter at Markes International, where he draws on the expertise of his colleagues to explain how new thermal desorption and mass spectrometry technologies can be applied to analyse volatile organic compounds in a wide variety of situations.

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